Shifting and binary bitwise operators applies on boolean arguments

Question

Python documentation for shifting operations and binary bitwise operations says that arguments must be integers, but the below expressions evaluates without error, however giving odd results for << and >>.

Is there an additional place I should look for documentation of & etc. when using boolean arguments, or is there some good explanation for evaluation and results ?

True & False: False (class 'bool')
True | False: True (class 'bool')
True ^ False: True (class 'bool')
~ True: -2 (class 'int')
~ False: -1 (class 'int')
True << True: 2 (class 'int')
False >> False: 0 (class 'int')

Code:

# Python ver. 3.3.2

def tryout(s):
    print(s + ':', eval(s), type(eval(s)))

tryout('True & False')
tryout('True | False')
tryout('True ^ False')
tryout('~ True')
tryout('~ False')
tryout('True << True')
tryout('False >> False')

Bakuriu Bakuriu · Accepted Answer · 2013-08-22T12:03:20

bool is a subclass of int, hence they are integers. In particolar True behaves like 1 and False behaves like 0.

Note that bool only reimplements &, | and ^(source: source code at Objects/boolobject.c in the python sources), for all the other operations the methods of int are used[actually: inherited], hence the results are ints and the semantics are those of the integers.

Regarding << and >>, the expression True << True is equivalent to 1 << 1 i.e. 1 * 2 == 2, while False >> False is 0 >> 0, i.e. 0 * 1 == 0.

You should think python's True and False as 1 and 0 when doing arithmetic operations on them. The reimplementation of &, | and ^ only affect the return type, not the semantics.

Shifting and binary bitwise operators applies on boolean arguments

1 Answers